1. Field of the Invention
The present invention relates to a broadcast receiver, and more particularly, to a carrier recovery apparatus for use in an OOB (out-of-band) receiver for an Open Cable System.
2. Discussion of the Related Art
Recently, digitalization of broadcast media is under rapid progress throughout the world and a digital broadcasting of a ground wave broadcasting and a satellite broadcasting has begun also in the country. In the county where more than 60% of the total television (TV) reception households are receiving a broadcasting through a wired network currently, an introduction of a digital system for a wired broadcasting is under active progress.
The Ministry of Information and Communication has propelled digitalization of a wired broadcasting since 1999 and selected an Open Cable Standard, which is an United States' standard, as a domestic digital wired broadcasting standard in 2001.
In the United States, the wired broadcasting occupies most largest portion in the whole broadcasting market but an equipment market for a related head end equipment and a set-top box is almost monopolized by two companies, which are Motorola, Scientific Atlanta. Thus, the United States is closed in viewpoint of the equipment market. To remove such closeness, the Unites States has prescribed that a part related to a limited reception should be separated from a set-top box from the coming year of 2005.
The Open Cable Standard prescribes that the set-top box should include a host from which a limited reception part is removed and a Point of Deployment (POD) including the limited reception part to meet Federal Communications Commission (FCC) regulations.
The Open Cable Standard requires the following functions:
First, a function for supporting FCC requirements, which include possibility of separating the limited reception function, purchasing of a subscriber terminal at a competitive market, compatibility with the existing digital wired broadcasting system.
Second, a function for supporting a performance parameter so that a network operation and audio and video qualities reach an acceptable level.
Third, a function for supporting a limited reception by a detachable POD.
Fourth, a function for supporting an out-of-band signaling. For Electronic Program Guide related service information, an Entitlement Management Message (EMM), a database, a predetermined band should be used.
Fifth, a function for supporting a high definition(HD) TV compression signal stream output using an Institute of Electrical and Electronics Engineers (IEEE) 1394 connection.
Sixth, a function for supporting a duplication meeting Motion Picture Association of America (MPAA) requirements for a higher level program.
Beside, a function for supporting a download and an application program.
Further, a matching standard roughly includes three kinds, which are a cable network matching, a matching of a subscriber terminal with a POD, and a matching of a subscriber terminal with an external apparatus. The cable network matching prescribes a signal transmitted and received at a subscriber terminal through a transmission line.
The matching of the subscriber terminal with the POD prescribes a matching signal between the subscriber terminal and the POD, and the matching of the subscriber terminal with the external apparatus prescribes a matching signal between the subscriber terminal and the external apparatus such as a TV and a video cassette recorder (VCR).
The cable network matching accepts most of “Digital Cable Network Interface Standard (SCTE 40 2001)”, which is a Society of Cable Telecommunication Engineer (SCTE) standard. This matching prescribes a signal between a subscriber terminal and a cable TV network in viewpoints of a physical hierarchy characteristic, a transfer hierarchy characteristic, and service and a related protocol stack.
The Open Cable Standard prescribes transmission of service/system information of a scrambled digital channel, an EMM, data through an out-of-band channel and prescribes that SCTE 40 (2001) should use either a Digital Video Standard (DVS)167 or DVS 178.
The out-of-band transmission should be performed in both directions and both a downlink and an uplink use a quadrature phase shift keying (QPSK) modulation. A transmission speed supports 1.544 Mbps, 2.408 Mbps, 3.088 Mbps for the downlink and 0.256 Mbps, 1.544 Mbps, 3.088 Mbps for the uplink. A use frequency supports 70˜130 MHz for the downlink and 5˜42 MHz for the uplink. An RF channel bandwidth supports 1.0/1.5/2.0 MHz for the downlink and 0.192/1.0/2.0 MHz for the uplink.
FIG. 1 is a view illustrating a structure of a carrier recovery loop of a related art OOB receiver.
As illustrated in FIG. 1, the carrier recovery includes a feedback loop. If a carrier passes through a phase detector 1 for compensating a static phase, a final demodulation output comes out.
If this final demodulation output passes through a slicer 2, a reference signal is obtained. A phase error detector 3 calculates a phase error using this reference signal.
A phase error calculated by the phase error detector 3 is accumulated by a loop filter 4. The loop filter 4 includes a loop filter for phase 4-1 and a loop filter for frequency 4-2.
If the loop filter for phase 4-1 estimates a static phase, the phase detector 1 compensates a phase error and makes an original value. At this point, the loop filter for phase 4-1 is a short loop having a very short group delay.
The loop filter for frequency 4-2 compensates for an obtained carrier frequency offset at a front end of a demodulator. Unlike the phase detector 1, the loop filter for frequency 4-2 has a considerably long group delay. In spite of such a long loop, the loop filter for frequency 4-2 compensates for the frequency offset with a stable performance.
As described above, the loop filter 4 receives a phase error from the phase error detector 3 for its input and multiplies the phase error by a loop bandwidth and performs an accumulation. The loop bandwidth should be scaled down as time goes on, so as to converge to an accurate direction. That is, it is important to set the bandwidth in a direction that a signal-to-noise ratio (SNR) is increased.
At this point, an element for changing the bandwidth step by step as time goes on is a lock detector 7. Though there exist various lock detectors, a lock detector whose performance is perfect does not exist substantially because reaction pattern of the lock detector is different depending on channels.
Since it is substantially difficult to design the lock detector satisfying all conditions, the lock detector is designed to satisfy a considerably flexible condition so as to converge up to the last bandwidth under most of conditions, though there exists a time difference.
FIG. 2 is a graph of a shape of the frequency offset with respect to time, estimated by the loop filter, which is an index representing loop filter's characteristic.
As times goes on, a frequency offset converges to a carrier frequency offset sought for as illustrated in FIG. 2. Generally, when the frequency offset converges to a correct direction, it is observed that the frequency offset converges to the shape as illustrated in FIG. 2. However, there occurs from time to time a case that the loop filter cannot find an actual frequency offset, converging to a false point.
Particularly, such a problem is generated for a quadrature phase shift keying (QPSK) type rather than a quadrature amplitude modulation (QAM) type. A reception of the current OOB system has a QPSK structure, such a phenomenon is often generated.
More specifically, in a process for obtaining the phase error, a vague phase is generated upon slicing. With such a circumstance, the QAM type does not make such a decision as to change a sign of the phase but the QPSK type often makes such a decision as to change a sign of the phase.